1. Field of the Invention
The invention relates in general to an aluminum (Al) conductive layer, and more particularly to a hillock-free Al layer and a method of forming the same.
2. Description of the Related Art
As the semiconductor manufacturing of an integrated circuit (IC) with larger scale is required, a substrate may be insufficient to provide an enough area for forming required interconnects for the IC. In order to meet the requirement of the formation of increased numbers of interconnects due to the reduction of metal oxide semiconductors (MOSs) of the IC in sizes, two or more levels of metal layers for interconnects have become a necessary technology adopted in the manufacturing of many ICs. Particularly, for some integrated circuits with sophisticated functions such as microprocessors, four or five levels of metal layers are required to implement interconnections of the components of the integrated circuits. On the other hand, in the manufacturing of a thin-film transistor liquid crystal display (TFT-LCD) panel, the metal films are employed as electrodes and interconnects, which are also formed in a structure with multiple layers of metal films.
In a structure with multiple layers of metal films, there are insulating layers, such as dielectrics, formed between any two of the metal layers in order to prevent an interlayer short circuit from occurring. In addition, a pure metal or an alloy with low electric resistance is suitably used as the material for the metal layers. In general, for examples of pure metals, Cr, Al, Cu, Mo, Ta, and W can be used. As examples of alloys with low electric resistance, an aluminum alloy containing one or more selected from the other elements, such as Al—Cu, Al—Cu—Si, Al—Pd, and Al—Nd, is used. Preferably, pure aluminum is employed as the material for metal layers. It is because aluminum has considerable adhesion with the substrate, and considerable etching characteristics in manufacturing as well as low electric resistivity. Besides, the earth contains much aluminum than other metal elements. Thus, aluminum is available and low in cost.
However, it still has disadvantages to use pure aluminum, which has a melting point lower than other metals, as the element for metal layers. Referring to FIG. 1A, it illustrates the deposition of a metal on a glass plate. In the manufacturing of thin film transistors, firstly, grains 104 are formed on a glass plate 102 by the deposition of metal under relatively low temperature (about 150° C.) and grain boundaries 106 are formed between the grains. In fact, the grains will not formed regularly in the same way as shown in FIG. 1A and the regular grains shown in FIG. 1A are for the sake of illustration. Next, annealing is performed so that the increased vibration of the grains by heating at high temperature causes the re-arrangement of the atoms of the grains, thereby reducing defects of the grains and re-crystallizing the grains. After the re-crystallization, inner stress of the grains is rapidly reduced by the reduction of the density of defects such as dislocation. If the annealing temperature is being increased and raises the grains formed in the re-crystallization to a higher energy level exceeding the surface energy among the grains, the grains begin to grow while the smaller ones of them vanish. Consequently, the growth of the grains yields larger grains and the grain boundaries of the smaller grains vanish. Thus, the inner stress of the grains is further reduced to a lower level.
When pure aluminum is used as the wiring layer material, hillock and the like may be produced. FIG. 1B shows the hillock by illustrating the glass plate with pure aluminum as the wiring layer material after annealing. In the annealing, the high temperature causes the thermal expansion of Al grain 104 and glass plate 102. Since aluminum has a greater thermal expansion coefficient than the glass, a substantial compressive stress by the Al grain 104 is applied to the glass plate 102. By this compressive stress, the aluminum atoms move along grain boundary 106 to cause a hillock 110. The hillock and the like, such as the hillock 110, may cause the unevenness of the thickness of the other layers in the subsequent fabrication process. Besides, in the worse case, an interlayer short circuit may occur when a large hillock penetrates an insulting layer (not shown) to be formed between the underlying metal layer and the overlying metal layer, and touches the overlying metal layer.
Hence, it is necessary to solve the problem of hillock in order to use Al as the wiring material. Conventionally, there are two approaches to this problem. The first approach is to use the other element having a high melting point, such as Nd, Ti, Zr, Ta, Si, and Cu, as the wiring material. FIG. 2A shows that grains 204 of an Al alloy formed on a glass plate 202 after annealing. As shown in FIG. 2A, there is no hillock formed among grain boundaries 206 of the grains 204 of the Al alloy. Since the atoms of the additional element of the Al alloy cannot dissolve in Al grains, as the grains 240 grow, the atoms of the additional element move to the grain boundaries 206 and gradually form small particles 210 among the grain boundaries 206. Thus, when Al atoms move along the grain boundaries 206, the small particles 210 hinder the Al atoms from moving above the grains 204, suppressing the formation of hillock.
The second approach is to form a metal layer with high melting point covering the Al grains so as to suppress the growth of hillock. FIG. 2B illustrates a metal layer capping the Al grains. After a metal layer 212 with a high melting point is plated over the Al grains 204, annealing is performed. Since the metal layer 212 works as caps for covering the exits formed by the grain boundaries 206 among the Al grains 204, Al atoms are blocked from forming hillocks along the grain boundaries 206. In addition, there is provided with a variant of the second approach where an Al layer in an amorphous phase is substituted for the metal layer 212. And Al layer in an amorphous state can be formed on the grains 204 for the suppression of the formation of hillock.
For these convention approaches to the problem of forming hillocks, it is the first one that is the most effective and usually employed. For example, a Japanese company, Kobelco, provides an Al—Nd alloy as the wiring material for metal layers, which is described in U.S. Pat. No. 6,033,542 to Yamamoto, et al. Nd has a large atomic weight and a high melting point, so that Nd can form small particles to hinder Al atoms from moving along the grain boundaries and forming hillocks. In this way, hillocks do not occur even if the temperature reaches 300° C. However, manufacturing cost is increased because Nd is a rare earth element, and it is required to apply a low sputtering rate in order to avoid splashing. Besides, Nd has a high resistivity so that an Al—Nd alloy has a resistivity higher than that of the pure aluminum.
As described above, the use of Al as wiring or electrode material in general semiconductor and liquid crystal display manufacturing is desired so that the study of the prevention of generating hillocks when Al is used therein is of great significant.